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Another fun way to use JavaScript on

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web pages is to animate properties over

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time. Before we can do that however, I

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need to make sure that you know about

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another global variable that is available

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on every web page, the <i>window</i>

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variable. Here, I will

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<i>console.log(window)<i> and you can</i></i>

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pause, check out your dev tools and see

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what's inside it.

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Did you see? It is huge. It contains so

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much, it is overwhelming. To make it a

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little less overwhelming I will tell you

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some of my favorite properties and methods

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that you can access on it.

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There is <i>window.location</i> which has

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a bunch of information about the URL of

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the page. Actually, let's go and just

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write that out to the page so that you

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don't have to keep pausing. So

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<i>textContent += "The URL of this page</i>

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<i>is " + window.location</i> and then that's

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an object, so we need to reach inside and

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say <i>.href</i> and there you go. That's

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actually the URL of the iFramed web page

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on this side.

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Another property, is <i>window.navigator.userAgent</i>

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which tells you about the browser of

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the user. Say <i>"The user agent is " +</i>

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<i>window.navigator.userAgent;</i>. Okay,

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that <i>userAgent</i> string probably looks

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pretty crazy to you. It's not really meant

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to be human readable and it doesn't

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always make a lot of sense for historical

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reasons. So, most web developers use

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libraries to actually understand what that

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string means and what browser they're on,

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and what OS and all that, because it is

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very weird. Here is one that's not so

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weird, <i>window.outerWidth</i> and <i>window.outerHeight</i>.

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So let's say, <i>"This web page is " +</i>

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<i>window.outerWidth + " by " + window.outerHeight;</i>

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To me it says, 1280 by 715, but maybe it

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says something different to you depending

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on what your page looks like when you are

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viewing this talk-through.

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Now, let me show you something a little

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surprising. I'm going to delete, the

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<i>window</i> part of this line of code.

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What happens is that it still works, that

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is because <i>window</i> is the default

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global variable on web pages. When the

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browser looks for a variable that you use,

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it will look for it on the <i>window</i>

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object. And when you create a new global

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variable, the <i>window</i> object will

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actually store it as a property. That

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means you should not declare your own

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variables named <i>outerWidth and outerHeight</i>

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because then they'll override <i>window.outerWidth</i>

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and <i>window.outHeight</i>. Generally,

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you should avoid global variables all

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together since it is so easy for them to

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collide with each other or existing variables

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on the <i>window</i>. To be extra safe, you

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can prefix your global variables. Like at

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Khan Academy, we write <i>KA_</i> in front

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of any global variables that we have to have.

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Okay, so that's the <i>window</i> object,

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now we are going to see how you can use

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two functions on it to make animations.